Trees affected by pine-oak gall rust

Pine-oak gall rust is a native fungal disease caused by Cronartium quercuum f.sp.banksianae. Two different host plants are required to complete the pathogen's life cycle. One plant from the pine family (Pinaceae) and the other from the oak family (Fagaceae).

Trees commonly affected by pine-pine gall rust include:

Where do pine gall rusts occur in Minnesota?

On Jack pine trees, pine-pine gall rust is most common in the northernmost counties of Minnesota including Cook, Lake, St. Louis, Koochiching, Lake of the Woods, Roseau, Itasca and Beltrami.

Pine-oak gall rust is most common in counties south of those listed above where red oaks are present.

How do pine gall rusts survive and spread?

Gall girdling a young pine tree

Pine-oak gall rust life cycle

Pine-oak gall rust is caused by Cronartium quercuum f.sp.banksianae.

In spring or early summer, airborne spores are released from last year’s infected oak leaves.

These spores infect young needles and shoots of pine trees.

The fungus moves from the needles into branches or stems and stimulates the tree to produce more and unusually large cells. This causes a gall to form.

Water and nutrients don’t move easily through the gall. As a result, needles beyond the gall may turn yellow or brown and the branch eventually may die.

Infected branches become brittle and snap off from extreme winds or snow loads.

Galls are not usually visible for several months to a year after infection.

When the gall reaches maturity, the bark on the gall cracks and flakes off, releasing bright yellow-orange, powdery spores in spring.

These spores infect the leaves of nearby oak trees.

During the summer, clumps of powdery orange spores develop on the underside of oak leaves. These spores can only infect other oak leaves.

Later in the season, dark brown-to-black, hair like structures appear on the underside of leaves on the infected oak. These specialized spores, called telia, produce a different spore type that infects nearby pines the following spring.